Limit numpy to one thread in heat equation

[dafeda]

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure tests pass locally (after every commit!)

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.88%. Comparing base (9ccd528) to head (d375a85).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8429   +/-   ##
=======================================
  Coverage   90.88%   90.88%           
=======================================
  Files         347      347           
  Lines       21123    21123           
=======================================
  Hits        19198    19198           
  Misses       1925     1925           

Flag	Coverage Δ
gui-tests	76.16% <ø> (ø)
integration-tests	54.24% <ø> (-0.02%)	⬇️
unit-tests	68.93% <ø> (ø)

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[sondreso]

Nice find! 🕵️